Method and apparatus for floatingzone melting of semiconductor rods



July 11, 1961 KELLER 2,992,311

W. METHOD AND APPARATUS FOR FLOATING-ZONE MELTING OF SEMICONDUCTOR RODSFiled Sept. 28, 1960 3 Sheets-Sheet 1 T A)I(IAL SHIFT 1 14/ l I 15LATERAL 12 6a 15 SHIFT OF SENSORS- SIIJEED 10 5 CONTROL GEN.

IAXIAL SHIFT Fig. 3

July 11, 1961 w. KELLER 2,992,311

METHOD AND APPARATUS FOR FLOATING-ZONE MELTING OF SEMICONDUCTOR RODS Z5Sheets-Sheet 2 Filed Sept. 28, 1960 July 11, 19

Filed Sept. 28, 1960 KELLER W. METHOD AND APPARATUS FOR FLOATING-ZONEMELTING OF SEMICONDUCTOR RODS 3 Sheets-Sheet 3 Fig. 6

Fig. 7

United States Patent G 2,992,311 METHOD AND APPARATUS FOR FLOATING- ZONEMELTING OF SEMICONDUCTOR RODS Wolfgang Keller, Pretzfeld, Germany,assignor to Siemens Schuckertwerke Aktiengesellschaft, Erlangen,Germany, a corporation of Germany Filed Sept. 28, 1960, Ser. No. 58,94817 Claims. (Cl. 219-10.77)

My invention relates to a method and apparatus for the crucible-freezone melting of semiconductor material according to which asemiconductor rod is kept Vertical between two holders, and a meltingzone, produced by an induction coil surrounding the rod is movedlongitudinally of the rod.

It has become known to vary during zone melting the axial spacingbetween the holders to which the rod ends are attached, so as tomaintain a constant diameter of the rod being processed. It has alsobeen proposed to control such change in axial spacing in dependence uponthe magnitude of the alternating current passing through the inductanceheater coil, this current being dependent upon the degree of inductivecoupling between coil and melting zone and hence also upon the diameterof the zone. This method affords producing a zonemelted rod of strictlydefined cross section which is constant over the processed length of therod.

Such current-responsive regulation, however, is dependent, forsatisfactory performance, upon certain conditions and limitations. Forexample, when the diameter of the rod is very small, such as only a fewmillimeters, the changes in heating current due to diameter variationsof the melting zone are too slight for accurate regulating performance.Difiiculties are also encountered when the zone melting is to beperformed in such a manner as to thereby change, particularly reduce,the diameter of the original rod.

It is an object of my invention, therefore, to devise an improved methodand apparatus capable of performing or facilitating the crucible-freezone-melting operation with reliable accuracy regardless of themagnitude of the rod diameter or for the purpose of changing thediameter.

To this end, and in accordance with a feature of my invention, I producea preferably magnified image or shadow picture of a rod portion whichcomprises the transition of the melting zone to the newly frozensemiconductor material, such image being produced by means of optical orother radiation. I further provide radiation sensing means at thelocation of the image for providing a variable voltage in response todimensional changes of the image and hence of the zone diameter, and Icontrol the axial spacing between the ends of the semiconductor rod independence upon the sensor voltage thus obtained.

According to another, more specific feature of the invention I providetwo radiation sensors horizontally spaced from each other a normallyfixed distance corresponding to the desired diameter of the resolidifiedsemiconductor material, the horizontal line of mutual spacingintersecting the image of the resolidifying semiconductor material inthe immediate vicinity of the melting zone. I further jointly displaceboth sensors along this line, preferably by an automatic regulatingdevice, so as to keep one sensor located on the image edge of therecrystallizing semiconductor material; and whereas I use the othersensor for regulating, preferably by another automatic device, thediameter of the recrystallizing semiconductor material so as to maintainthe second sensor located on the other image edge of the recrystallizingsemiconductor material.

The invention is particularly advantageous for changing the crosssection of a semiconductor rod by zone melting to a desired, differentvalue. This advantage is particularly conspicuous when reducing thecross section of the semiconductor rod because it may happen in suchcases that the reduced rod portion has too slight a reaction upon theheating current supplied to the inductance coil, so that this effect canno longer be employed for regulating purposes as is the case in theknown method mentioned above.

In the following, therefore, the invention will be particularlydescribed with reference to the performance of the novel method or thereduction in cross section of the semiconductor rod, although the methodis equally well applicable for other purposes, such as simply processing a semiconductor rod without change in cross section, particularlythe processing of very thin rods.

For various purposes the semiconductor rods to be zone melted must havea very small diameter, although, as a rule, the material originallyproduced has much greater thickness. For example, according to a knownprocess, semiconductor material, such as silicon, is produced bychemical decomposition of a gaseous compound which is precipitated upona filament or core rod for example of 2 to 5 mm. diameter, of the samematerial until the original core with the precipitated material hasgrown to a semiconductor rod of the usual thickness, such as 10 to 20mm., which can readily be subjected to the known zone-melting methods.The production of the filaments or cores involves various difficultieswhose elimination constitutes another object of the present invention.It is particularly diflicult to secure a uniform diameter of these thinproducts, and it is therefore a more specific object of the invention toreliably secure such uniformity. Among other purposes of relatively thinsemiconductor rods is the production of seed crystals to be employed forgrowing monocrystalline products from polycrystalline materials.

The foregoing and other objects, advantages and features of myinvention, said features being set forth with particularity in theclaims annexed hereto, will be apparent from, and will be described in,the following with reference to the embodiments of apparatus accordingto the invention illustrated by way of example on the accompanyingdrawings in which:

FIG. 1 illustrates, on enlarged scale, the portion of a semiconductorrod in which a reduction in cross section is to be produced by zonemelting.

FIG. 2 shows a top view of a portion of apparatus according to theinvention corresponding substantially to a horizontal section throughthe upper portion of FIG. 1 along the line 7 and supplemented by amagnifying lens.

FIG. 3 is a block diagram of apparatus for performing the methodaccording to the invention.

FIG. 4 illustrates details and an electric circuit diagram of theapparatus according to FIG. 3.

FIG. 5 is a schematic diagram of a modified apparatus according to theinvention.

FIG. 6 illustrates an electric circuit diagram of apparatus according toFIG. 5, and a schematic representation in block fashion of thosecomponents that are identical with those shown more in detail in FIG. 4.

FIG. 7 illustrates another modification of apparatus according to theinvention with a block-fashion illustration of the components more fullyshown in FIG. 4.

Shown in FIG. 1 is a portion 2 of a rod consisting, for example, ofsilicon, germanium, indium arsenide, indium antimo-nide or othersemiconductor substance. The originally thick rod is to be reduced to aportion 3 of a given smaller diameter d. Located between the two rodportions is a melting zone 4 which is produced by means of an inductancecoil consisting preferably of a flat spiral made of copper tubing whichis traversed by cooling water when in operation. Further shown in FIG. 1are two radiation sensors 6a and 6b, such as photoelectric cells orthermocouples. These are shown mounted along a horizontal line 7extending at a right angle to the rod axis. The spacing between the twocells 6a and 6b is shown to be equal to the diameter d of the thin rodportion 3. Actually however, the cells 6a and 6b are not locateddirectly at the semiconductor rod, but are horizontally spaced therefromso as to respond to an image of the rod. The horizontal line 7intersects the semiconductor rod or its image at a right angle to thetravelling motion of the two rod-holders" to which the respective rodends are attached. The line 7 is located in the direct vicinity of themelting zone or its image at the location where the liquid materialresolidifies and forms the thinner rod diameter.

It is preferable to produce a magnified image of the rod particularlywhen the diameter d is very small. For this purpose, according to FIG.2, a magnifying lens 8 or lens system is provided. The sensors 60 and 6bare located on a line 711 which corresponds to line 7 in FIG. 1 but islocated in the plane of the magnified image. The vertical spacing ofline 7 or 7a from the plane of the flat heater coil 5 may either beadjusted to a fixed value or may be made variable. In the first case, acertain safety distance between the upper boundary of the melt and thereference line 7 or 7a must be preserved so that minute variations inshape and size of the melting zone as may occur by the regulatingoperation and other minor effects, will not have the result that theradiation sensors become located in front of the melting zone 4 insteadof in front of the tin rod portion 3'. When operating with variablevertical spacing between melting zone and reference line, a furtherradiation sensor may be provided for controlling a device whichregulates the vertical spacing between reference line and heater coil bymaintaining the line 7 or 7a always coincident with the upper boundaryof the melting zone 4 or its image. In the following explanations, it isassumed that the reference line 7 or 7a has a fixed vertical spacingfrom the plane of the heater coil 5.

FIG. 3 shows the same semiconductor rod vertically held between holders2a and 3a. A capacitor 9 is connected parallel to the induction coil 5for compensating the reactive (wattless) current. The oscillatorycircuit (heating circuit) thus formed is energized from a highfrequencygenerator 10. The frequency of the generator is preferably so adjustedthat it is almost, but not quite, identical with the natural frequencyof the heating circuit 5, 9 so that the generator operates on a flank ofthe resonance peak in the current-versus-frequency characteristic of theheating circuit. This permits adjusting or regulating the current incoil 5 by correspondingly changing the frequency tuning of thegenerator.

The radiation sensors 6a and 6b, consisting for example of photoelectriccells or thermocouples, are firmly mounted on a carrier 11 at a normallyfixed but ad justable horizontal distance from each other. The carrier11 is displaceable horizontally along the line 7a (FIG. 2) by means of ashifting device 12. Such horizontal shifting of carrier 11 is controlledby the sensor 6a in such a manner that this sensor is always located atthe left margin of the image formed by the thin rod portion 3. Suchautomatic displacement regulation of carrier 11 can be carried out, forexample, by having the thin rod portion 3 form a bright image on a darkbackground and having the radiation sensor 6a control the shiftingdevice 12 so as to move the carrier 11 to the right when the sensor 611is located on a dark area, and to move the carrier 11 to the left whenthe sensor 6a is located on a bright area and hence within the image ofthe thin rod portion 3. The circuitry used for this purpose ispreferably such that the shifting device 12, with the carrier 11 and thetwo sensors 6a and 6b, is at standstill when the sensor 6a is located onsuch a gray value as will obtain at the edge of the image formed by thethin rod portion.

For simplicity of illustration it is disregarded in FIG. 3 (also in FIG.5) that the transition from the thicker to the thinner rod portion isactually represented by an enlarged image and hence that the axialspacing between the sensors 6:: and 6b is larger than apparent from FIG.3 (or FIG. 5). This is more accurately illustrated in FIGS. 2 (and 4).Due to the magnified image, the regulating accuracy of the devicesdescribed is increased accordingly.

The radiation sensor 6a, here shown to consist of a single cell, mayactually comprise two photocells closely beside each other of which oneis normally located on the image of the semiconductor rod (bright),whereas the other component cell must be located on the background(dark) in order to keep the shifting device 12 with carrier Ill andsensors 6a, 6'0 at standstill. The radiation sensor 6b can be given acorresponding design. In this case, the image edge of the thin rodportion must always be located accurately between the two photocells.

During zone-melting operation, the lower, thick rod portion 2 is kept inslow upward motion by means of an axial shifting device 13.Simultaneously the thin rod portion 3 is likewise shifted upwardly by anaxial shifting device 14 but at greater speed in accordance with thereduced cross section of the rod.

The sensor 611 furnishes its measuring quantity to a speed controldevice 15 by means of which, through a connection 16, the speed of theupper axial shifting device 14 can be varied. When the sensor 6b islocated on dark area, the travelling speed of the thin rod portion 3 isreduced so that thickening of the melting zone and hence of this rodportion will result. When the sensor 51) is located on the bright imageof the thin rod portion 3, the speed of the axial shifting device 14 isincreased so that the upward travel of the thin rod portion iscorrespondingly accelerated in order to reduce the cross section of themelting zone and hence of the thin rod portion 3. As described abovewith reference to the sensor 6a for control of the lateral jointshifting motion of both sensors, the circuitry of sensor 612 ispreferably such that this sensor will not issue any speed-changecommands when it is located in the gray region corresponding to the edgeof the image representing the thin rod portion 3.

As will be more fully explained below, the increase and reduction intravelling speed of the holder 3a to which the thin rod portion 3 isattached can be effected, for example, by switching an auxiliaryvoltage, variable in magnitude and direction, upon the drive motor ofthe upper axial shifting device 14.

During zone-melting the amount of power supplied to the heating circuit5, 9 and hence to the melting zone is kept constant by having thehigh-frequency generator 10 operate at a constant frequency andmaintaining the output or plate current of the generator at a constantvalue. This can be effected, for example, by causing the load circuit ofthe generator to produce a corresponding voltage drop in a resistorserially connected in the load circuit, and comparing this voltage dropwith an adjusted datum voltage so as to obtain a difierence voltagewhose polarity and magnitude depend upon the departure of the generatorload current from the desired datum value. This difference voltage canbe used for controlling a polarized relaying device to add an additionalpositive or negative voltage to the one normally supplied to the drivemotor of the lower axial shifting device 13. An action line 17 is shownin FIG. 3 to symbolize such control of the lower shifting device 13 bythe load current of the generator 10. With such a feedback control, anincrease or decrease in the degree of coupling between melting zone 4and heating coil 5 produced by the faster or slower travel of the thickrod portion 2,

has the efiect of increasing or decreasing the generator output currentin the sense required to maintain this current constant at the datumvalue.

Details of the above-described apparatus of FIG. 3 will be explainedpresently with reference to FIG. 4, the reference characters beingidentical with those used for corresponding elements in all otherillustrations.

As shown in FIG. 4, a lamp 8a and a lens 8 produce a magnified image ofthe melting-zone region as identified above with reference to FIG. 1,the contour line of the image in FIG. 4 being denoted by 8b. The twosensor cells 6a and 6b are mounted on the carrier 11 so that one cell ishorizontally adjustable relative to the other, for example by amicrometer screw (not illustrated). The carrier 11 is shown attached toa screw spindle 1101 which is horizontally displaceable but notrevolvable with respect to a fixed supporting or frame structure 1a ofthe apparatus. Horizontal displacement of spindle 12a is effected bymeans of its threaded engagement with the hub of a worm gear 12b whichmeshes with a worm 12c driven from a motor 12M. This motor is reversibleand is energized from a direct-current source 12d under control by twocontactors 12L and 12R. The two contactors are polarized by means ofdiodes so that contactor 12L will pick up and cause the motor 12M toshift the carrier 11 to the left when an amplifier 12e supplies negativeoutput voltage, whereas the contactor 12R picks up to make motor 12Mshift the carrier 11 to the right when the amplifier 12c suppliespositive output voltage. The amplifier output voltage is controlled bythe above-mentioned sensor cell 6a whose voltage is compared with anadjusted datum voltage taken from a potentiometer 12f energized from aconstant direct-voltage source 12g. Thus, the input voltage and hencealso the output voltage of amplifier 12c is either positive or negativedepending upon whether the sensor 6a is subjected to bright or darkareas of radiation or illumination as explained above. As long as thesensor 6:: is in the marginal gray area, neither contactor is energizedso that the motor 12M is at rest. In this manner the lateral shiftingdevice 12 controls the horizontal position of carrier 11 so as to keepthe sensor 6a properly located at one edge of the image.

The axial shifting device 13 for the lower rod-end holder 2a comprises areversible motor 1 3M. The holder 2:: is rigidly connected with avertical screw spindle 13a which is longitudinally displaceable but notrevolvable in the rigid supporting or frame structure 1a and is inthreaded engagement with a worm gear 13b meshing with a worm 13c drivenfrom motor 13M.

The upper holder 3a is similarly connected with a vertical screw spindle14a which is axially displaceable but not revolvable in supportingstructure 1a and is operated from a reversible motor 1 4M by means of aWorm gear 14b in mesh with a worm 140.

The circuits of motors 14M, 13M and of the generator comprise gangedswitch contacts S1, S2, S3 which are to be closed when the apparatus isto be put into operation.

The motor 14M of the upper axial shifting device 14 is energized from adirect-current source 14d under control by two contactors 15S and 15FWhose respective contacts connect to the motor circuit an auxiliaryvoltage of negative or positive polarity relative to the voltage of mainsource 14d. The auxiliary voltage is taken from a potentiometer 15eenergized from a source 157 of constant direct voltage. When contactor158 is energized, it causes the motor 14M to reduce its speed to a valueadjusted at potentiometer 15a. When contactor 15F picks up itcorrespondingly increases the speed of motor 14M in accordance with avalue adjusted at potentiometer 15e. Energization of the contactors 15Sand ISP is controlled by polarized relay means here shown as anelectromagnetic relay 15P. This relay is controlled by the putputvoltage o the second radiation sensor 612 in differential relation to areference voltage tapped ofi' a potentiometer 15g energized from adirect-voltage source 15h. It may be mentioned at this point that while,for simplicity of illustration, separate current sources areillustrated, they can all be supplied from a single power supply or asuitable group of supplies, preferably energized from an availablealternating-current or direct-current utility line.

When the voltage from sensor 6b is above the datum value set atpotentiometer 15g one of contactors 15S, 15F will respond, and when thesensor voltage is below the datum value, the other contactor willoperate. As a result, the motor 14 is speed controlled in dependenceupon whether the sensor 6b is in front of a bright or dark area, asexpalined above. As long as the sensor 6b is located in the marginalgray zone of the image, the polan ized relay '15P is inactive and themotor 14M then runs at the normal speed. This speed is properly relatedto that of the motor 13M, and any suitable electrical or mechanicaltransmission means for securing a proper speed ratio between devices 13and 14 can be used if desired. For example, only one of motors 13M, 14Mcan: be used and the other motor be substituted by a step downtransmission gearing between devices 13 and 14,. having an adjustabletransmission ratio controlled by device 15 to clamp the speed ratiounder control by sensor 6b in the sense explained above.

As mentioned, the lower axial shifting device 13 is con" trolled independence upon the load current of the high-' frequency generator 10.This generator is shown to comprise an electronic triode 10T connectedwith a tank circuit which comprises a capacitor C and an inductance coilL. The coil forms the primary winding of a transformer which couples thegenerator 10 with the heating; circuit 5, 9. The load current of triode10T, coming from a current source 10a, passes serially through a resistor R which develops a voltage drop proportional to the load current.This load current is dependent upon the thickness of the melting zone asmentioned above.

The voltage drop of resistor R, and hence the amount of load current, isindicated by a measuring instrument 10b and is compared with a datumvoltage from a source of constant reference voltage. The resultingdiffer-- ence voltage is applied to suitable polarized relay means,again shown as a polarized electromagnetical relay 17P. Dependent uponwhether the generator load current is above or below the datum valuecorresponding to the voltage of source 100, the polarized relay 17Penergizes one or the other of two contactors 17S and 17F.

The motor 13M of the lower axial shifting device 13 is energized from acurrent source 13d which is normal ly alone effective to determine themotor voltage and hence the travelling speed of the lower rod holder 2a.However, when the current in the heating circuit increases due to thediameter of the melting zone becoming too large, this is sensed by thepolarized relay 17B which then energizes the contactor 17F with theeffect of corn necting into the motor circuit an auxiliary voltage takenfrom a potentiometer 17a which is energized from a current source 17band so poled as to increase the terminal voltage of the motor thusincreasing the travelling speed of the holder 2a which then pulls thetwo rod portions 2, 3 farther apart and thereby reduces the crosssection of the melting zone and returns the current to the proper value.

Conversely, when the load current of the generator decreases because thediameter of the melting zone drops below the correct value, thedeparture of the load cur rent from the datum value is sensed by thepolarized relay 17P which then energizes the contactor 17S. Contactor178 then connects into the circuit of motor 13M a voltage taken from apotentiometer 17c which is energized from a current source 17d and sopoled as to reduce the resultant voltage impressed upon the motor 13M,thus causing a reduction in travelling speed of the rod holder 24.

As a consequence, the heating current supplied to the coil is regulatedto remain substantially constant at the datum value determined by thevoltage of reference source c.

The apparatus illustrated in FIG. 5 is fundamentally similar to the onedescribed above with reference to FIGS. 3 and 4, with the exception ofthe following modifications. The variation in diameter of the thin rodportion 3 is effected, not by changing the speed of the upper axialshifting device 14- but by varying the frequency of the generator 10.The load current of the high-frequency generator 10, as in theembodiment of FIGS. 3 and 4, is kept constant by correspondingly varyingthe travelling speed of the thick rod portion 2, whereas the frequencyof the generator 10 is controlled by the sensor-responsive device 15 inthe desired sense through a controlling connection 18. In this case, thethin rod portion 3 is moved upwardly at constant speed. The frequencyvariation of generator 10 causes a corresponding variation in powersupply to the heater coil 5 and hence a corresponding change ingenerator load current. By change in upward travel speed of the thickrod portion 2, such a change in frequency is immediately eliminated. Theultimate effect, therefore, is that the diameter of the thin rodportion, travelling at constant speed relative to the heater coil whichreceives a constant power supply, is maintained at the desired value bycorrespondingly varying the travelling speed of the thick rod portion 2.

An example of details of the justmentioned modification will bedescribed with reference to FIG. 6, it being understood that thecomponents 12, 13, 14 and 17 in FIG. 6 may be identical with thosedescribed above with reference to FIG. 4.

According to FIG. 6, the motor 14M of the upper axial shifting device 14is energized by constant voltage from source 11d in order to move theupper rod holder at constant speed. The high-frequency generator 10 hasits tank circuit provided not only with the main capacitance C but alsowith auxiliary capacitors C1 and C2 of which the capacitor C1 isnormally connected parallel to main capacitance C through the contact ofa relay 18A, whereas the auxiliary capacitor C2 is normally disconnectedfrom the tank circuit under control by another relay 1813. When thepolarized relay 15P is in the normal position, namely when the sensor612 according to FIG. 4 is properly located on the gray margin of theimage, the frequency of the generator 10 is determined only by the twothin active capacitors C and C1. When relay 15P moves its contact fromthe inactive to one of its two active positions, depending upon whetherthe sensor 61) enters into the dark or bright range of the background orimage, one of the two relays 18A, 18B is selectively energized andthereby increases or decreases the total capacitance of the tank circuitin the generator, thus shifting the frequency above or below the datumvalue with the effect of varying the current of heater coil 5accordingly. This operation requires that the generator operate at afrequency on a flank portion of the currentversus-frequencycharacteristic of the heating circuit so that a change in generatorfrequency causes an increase or decrease of the generator load current.

The amount of load current causes a corresponding voltage drop in theresistor R, and this voltage drop is compared with an adjusted andnormally constant datum voltage taken from a potentiometer 10d energizedfrom a source 100 of constant reference voltage. As a result, thecontrol components of device 17 act upon the lower axial shifting device13 in the same manner and by the same means as explained above withreference to FIG. 4. The system therefore tends to maintain the diameterof the thin rod portion at the desired constant value by regulating thepower supply to the heater coil 5 to a constant value while varying thetravelling speed of the lower .rod portion.

The just-mentioned apparatus may be further modified by eliminating theconnecting devices 17 and thus the control of the lower axial shiftingdevice 13 by the load current of the generator 10. In this case, theonly change effected for regulating purposes is the change in generatorfrequency by means of the connecting devices 18, whereas the diameterregulation of the thin rod portion 3 is effected, at constant travellingspeed of both rod portions, exclusively by changing the power suppliedfrom the generator to the heater coil 5.

Still another possibility of regulation for the purposes of theinvention is to have the device 15 directly control the lower axialshifting device 13 and hence the travelling speed of the thick rodportion 2. Such a modification is illustrated in FIG. 7, it beingunderstood that the components 12., 13, 14 and 15 may be identical withthose described above with reference to FIG. 4.

As shown in FIG. 7, the speed control device 15 acts upon the motor 13Mso as to increase or decrease its speed in dependence upon the sensingoperation of sensor 6b. The motor MM is driven at constant speed, andthe frequency of the generator 1! is normally kept at a constant value.When thus operating with a constant travelling speed of the thin rodportion 3 and with a constant frequency of the generator 10, theregulating effect imposed upon the diameter of the thin portion isultimately again due to the fact that the power supply to the heatercoil 5 is controlled by correspondingly increasing or decreasing thesize of the melting zone with the aid of a corresponding change in speedof displacement imparted to the lower rod portion.

It will be understood by those skilled in the art that in other respectsthe method and apparatus according to the invention may be carried outin conventional manner. For example, the semiconductor rod as well asthe components of the heating circuit and the carrier 11, as well asother parts of the equipment, can be located within a hell or otherrecipient which is evacuated during the zonemelting operation, and thisrecipient as well as the extraneous components of the apparatus may bemounted together on a common carrier to form a single transportableunit. Such and other modifications, particularly with respect tostructural details or circuitry, are readily available to those skilledin the art upon a study of this disclosure, without departure from theessential features of my invention and within the scope of the claimsannexed hereto.

I claim:

1. In a method of zone melting a semiconductor rod, in which the rod isvertically supported at both ends and a molten zone is formed in the rodby a surrounding induc tive heater coil energized by current from analternatingcurrent generator and the zone is caused to move relative toand lengthwise of the rod, the improvement comprising the steps offorming an enlarged image of a rod portion containing the transitionfrom the molten zone to the resolidifying rod material, sensing theedges of the enlarged image at two locations horizontally spaced fromeach other a fixed distance along a line intersecting the image of theresolidifying area at a right angle to the rod axis, jointly shiftingboth sensing locations horizontally so 'as to maintain one of saidsensing locations on the corresponding one edge of the image, andcontrolling the cross-sectional width of the molten zone in theresolidifying rod area so as to thereby maintain the second sensinglocation on the second edge of the image.

2. In a method of zone melting a semiconductor rod, in which the rod isvertically supported at both ends and a molten zone is formed in the rodby a surrounding inductive heater coil energized by current from analternating-current generator and the zone is caused to move relative toand lengthwise of the rod, the improvement comprising the steps ofprogressively increasing the axial spacing between the two rod ends soas to reduce the cross section of the rod axially along the molten zone,forming an enlarged radiation image of the transition 9 from the thickto the thin rod portion, said image representing the-molten zone and theadjacent resolidifying boundary region of the rod, sensing the edges ofthe enlarged image at two locations horizontally spaced from each othera fixed distance along a line intersecting the resolidifying region at aright angle to the rod axis, jointly shifting both sensing locationshorizontally so as to maintain one of said sensing locations on thecorresponding one edge of the image, and controlling the cross-sectionalwidth of the molten zone in the resolidifying rod area so as to therebymaintain the second sensing location on the second edge of the image.

3. Ina method of zone melting a semiconductor rod, in which the rod isvertically supported at both ends and a molten zone is formed in the rodby a surrounding inductive heater coil energized by current from analternating-current generator and the zone is caused to move relative toand lengthwise of the rod, the improvement comprising the steps ofprogressively increasing the axial spacing between the two rod ends soas to reduce the cross section of the rod axially along the molten zone,forming an enlarged radiation image of the transition from the thick tothe thin rod portion, said image representing the molten zone and theadjacent resolidifying boundary region of the rod, sensing the edges ofthe enlarged image at two locations horizontally spaced from each othera fixed distance along a line intersecting the resolidifying area of thethin rod portion at a right angle to the rod axis, jointly shifting bothsensing locations horizontally so as to maintain one of said sensinglocations on the corresponding one edge of the image, and varying theaxial spacing between the rod ends to thereby control the cross sectionof the resolidifying thin rod portion as required to maintain the secondsensing location on the second image edge of the thin rod portion.

4. In a method of zone melting a semiconductor rod, in which the rod isvertically supported at both ends and a molten zone is formed in the rodby a surrounding inductive heater coil energized by current from analternating-current generator and the zone is caused to move relative toand lengthwise of the rod, the improvement comprising the steps ofsimultaneously displacing the two rod ends axially in the same directionrelative to the heater coil at respectively different rates for reducingduring zone melting the cross section of the rod axially along themolten zone, forming an enlarged radiation image of the transition fromthe thick to the thin rod portion, said image representing the moltenZone and the adjacent resolidifying boundary region of the rod, sensingthe edges of the enlarged image at two locations horizontally spacedfrom each other a fixed distance along a line intersecting theresolidifying area at a right angle to the rod axis, jointly shiftingboth sensing locations horizontally so as to maintain one of saidsensing locations on the corresponding one edge of the image, andvarying the displacing speed of one of said rod ends to thereby controlthe cross section of the resolidifying thin rod portion as required tomaintain the second sensing location on the second image edge of thethin rod portion.

5. The zone melting method of claim 4, wherein constant current issupplied from the generator to the heater coil, and the variation indisplacing speed is applied to the end of the thin rod portion.

6. In the zone melting method of claim 4, wherein said heater coil formspart of a high-frequency tank circuit and said generator supplieshigh-frequency current of constant frequency to the coil, the steps ofapplying the variation in displacement speed to the thick rod portion soas to thereby maintain said current at a constant value.

7. In a method of zone melting a semiconductor rod, in which the rod isvertically supported at both ends and a molten zone is formed in the rodby a surrounding inductive heater coil energized by current from analternat- 10 ing-current generator and the zone is caused to moverelative to and lengthwise of the rod, the improvement comprising thesteps of progressively increasing the axial spacing between the two rodends so as to reduce the cross section of the rod axially along themolten zone, forming an enlarged radiation image of the transition fromthe thick to the thin rod portion, said image representing the moltenzone and the adjacent resolidifying boundary region of the rod, sensingthe edges of the enlarged image at two locations horizontally spacedfrom each other a fixed distance along a line intersecting theresolidifying region of the thin rod portion at a right angle to the rodaxis, jointly shifting both sensing locations horizontally so as tomaintain one of said sensing locations on the corresponding one edge ofthe image, maintaining the displacement speed of one of the rod endsconstant, and varying the current passing through the heater coil independence upon the departure of the second sensing location from thesecond image edge to thereby control the cross section of theresolidifying thin rod portion as required to maintain the secondsensing location on the second image edge of the thin rod portion.

8. In the zone-melting method of claim 2, wherein the heater coil formspart of a tank circuit, the steps of maintaining the end of the thin rodportion at constant axial travelling speed relative to the heater coil,and controlling the diameter of the thin rod portion by varying thefrequency of the generator along a flank of the resonance characteristicof the tank circuit.

9. The zone-melting method of claim 8, the step of maintaining the endof the thick rod at a constant axial travelling speed relative to theheater coil.

10. In the zone-melting method of claim 2, wherein the heater coil formspart of a tank circuit and the current source has a frequency on theflank of the resonance characteristic of the tank circuit, the steps ofcontrolling said cross-section of the thin rod portion by varying theaxial travel speed of the thick rod end relative to the heater coil soas to maintain the current intensity in the heater coil at a constantvalue.

ll. Apparatus for zone melting of a semiconductor rod, comprising tworod-end holders vertically spaced from each other and defining avertical processing axis, an axially fiat inductance heater coilsurrounding said axis, an alternating-current source connected to saidcoil for energizing said coil, drive means for providing relative travelbetween said coil and said holders whereby said coil is caused toproduce in said rod a melting zone travelling lengthwise of theprocessing axis, radiation means disposed in fixed relation to said coilfor producing an enlarged image of a rod portion containing thetransition from the molten zone to the resolidifying rod material, tworadiation sensors responsive to opposite edges respectively of the imageand having a carrier in common, said two sensors being horizontallyspaced from each other a normally fixed distance along a lineintersecting the resolidifying area of the image at a right angle to theprocessing axis, said carrier with said sensors being displaceable alongsaid line, first control means connected with said carrier forhorizontally displacing said carrier, one of said sensors beingconnected to said control means for causing it to horizontally displacesaid carrier so as to maintain said one sensor in fixed space relationto one of said image edges, means for regulating the cross-sectionalwidth of said zone, said second sensor being connected to saidregulating means for controlling it in dependence upon departure of saidsecond sensor from the second image edge so as to maintain said secondsensor on said second edge.

12. In zone-melting apparatus according to claim 11, said heater coiland said image-producing means being fixed, and said two rod-end holdersbeing jointly displaceable axially in the same direction for causingtravel of the melting zone relative to the processing axis.

13. In zone-melting apparatus according to claim 12,

1 1 said drive means having two component drives of respectivelydifferent speeds connected with said respective two holders forprogressively lengthening said vertical spacing, whereby the rod beingprocessed is reduced in diameter within the melting zone.

14. In zone-melting apparatus according to claim 13, said regulatingmeans comprising one of said component drives for varying the axialspeed of the appertaining one holder under control by said secondsensor.

15. Zone-melting apparatus according to claim 14, comprising currentsensing means responsive to the magnitude of current supplied to saidheater coil, and further control means connecting said current sensingmeans with said other component drive for varying the axial travelspeed'of the other holder under control by said current sensing means soas to maintain said current magnitude constant.

16. In zone-melting apparatus according to claim 15, said one componentdrive being connected with the lower one of said holders and said lowerholder being adapted 12 for attachment of the thick rod portion, saidother component drive being connected with the upper holder for the thinrod portion and having greater normal speed than said one componentdrive.

17. In Zone-melting apparatus according to claim ll, said current sourcecomprising a high-frequency generator, a tank circuit of which saidheater coil forms part, said regulating means comprising frequencyadjusting means in said generator for varying the generator frequencywithin a range corresponding to a flank portion of the resonancecharacteristic of said tank circuit, whereby the cross-sectional widthof the zone is controlled by variation of the generator frequency undercontrol by said second sensor.

References Cited in the file of this patent UNITED STATES PATENTS

